Standardization for the fourth Generation (4G) of cellular networks is on going under the name International Mobile Telecommunications Advanced (IMT Advanced). IMT Advanced promises the next generation mobile network with high data rates, seamless connectivity and mobile communication within heterogeneous networks.
For many applications, a short access delay plays a crucial role in providing a good end user performance. IMT Advanced latency requirements state that the one way radio access delay between the mobile terminal and the base station should be under 10 ms.
The Long Term Evolution (LTE) network defined by 3rd Generation Partnership Project (3GPP) provides improved bit rates with lower access delays as compared to the older technologies. For Release-8 user equipment units (UEs), the IMT Advanced delay target is reached if the user equipment unit is scheduled. If the user equipment needs to request resources, the delay target is not reached.
The access technology of LTE is based on Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink.
In the present context, the expression uplink is used for the transmission of signals from the user equipment to the base station while the expression downlink is used for transmission of signals in the opposite direction, i.e. from the base station to the user equipment. The base station may further in an LTE environment be referred to as an eNodeB, or eNB.
The resources are allocated either dynamically in 1 ms basis for the user equipments on both the downlink and the uplink or semi-persistently over a longer period than 1 ms. The scheduling of resources is done by the eNB and it takes into account the user equipment data buffer as well as radio propagation characteristics of each user equipment.
The standard uplink scheduling procedure in LTE is shown in FIG. 1. When new data arrives to the empty user equipment buffer or the data that belongs to a higher priority logical channel group than existing data, the user equipment triggers a Buffer Status Report (BSR) to report its buffer size. If the user equipment does not have uplink resources to transmit the buffer status report, it triggers a Scheduling Request (SR).
The scheduling request can be sent on a dedicated scheduling request channel (D-SR) or on the contention based Random Access Channel (RA-SR). Use of the dedicated scheduling request channel requires that the user equipment is uplink synchronized and that the user equipment has been assigned a scheduling request channel on the Physical Uplink Control Channel (PUCCH). The dedicated scheduling request resource is assigned with Radio Resource Control (RRC) protocol having a periodicity with current values of 5, 10, 20, 40 and 80 milliseconds in LTE Release-8. In LTE Release-9, even shorter values than 5 milliseconds, are possible. When the eNB has received the scheduling request, it can schedule the user equipment and transmit an initial grant. Using the initial grant, the user equipment can finally transmit the buffer status report with it.
In summary, before the user equipment is scheduled, multiple steps have to be taken. This increases the access delay in the uplink. When being in uplink synchronized, the scheduling request periodicity in PUCCH is one of the biggest contributors in delay increase. To obtain best performance of certain applications, the scheduling request periodicity should be selected to very short value.
In the 3GPP TS 36.321 MAC specification, V8.6.0, the scheduling request is pending from the time when it is trigged until the time when it is cancelled. The scheduling request is cancelled when uplink scheduling resources are available for a new transmission.
When the scheduling request is pending, during every subframe when the user equipment unit has valid scheduling request resources on PUCCH, the user equipment unit instructs the physical layer to signal scheduling request. This leads to the physical transmission of the scheduling request. In the example depicted in FIG. 1, the scheduling request periodicity is fixed to 5 ms and the first opportunity to transmit the scheduling request on PUCCH is in subframe t0. After the eNB has received the scheduling request, a typical processing time of 3 ms is assumed before the user equipment unit is scheduled and the grant is transmitted. The user equipment unit has a next scheduling request opportunity in 5 ms later, at t1. Because the user equipment unit has not cancelled the scheduling request yet, it will retransmit it. As a result, configuring the scheduling request periodicity to 5 ms leads to physical transmission of the scheduling request at least twice. This generates a significant unnecessary load on PUCCH.
The mechanism that the transmission of scheduling requests is prohibited is known as well as configuration of such mechanism by the network, see WO 2009038381 A2 METHOD OF RESTRICTING SCHEDULING REQUEST FOR EFFECTIVE DATA TRANSMISSION. However, in WO 2009038381 A2, the only mentioned condition when the transmission of scheduling request is prohibited is when the uplink synch, i.e., Time Alignment timer is expected to expire soon or when the overall number of scheduling request transmissions has exceeded a certain number. The idea of WO 2009038381 A2 is instead to start random access if scheduling request is prohibited.